Staunt U.S. Pat. No. 3,325,899 discloses a wrench having a two-piece body, one section 32 of the body having a central shaft 31 of non-circular cross section secured thereto and the other section 33 being provided with a plurality of fixed pins 40 for locking the rotor and cap of a handpiece together when the wrench is in use. Once the lower section 33, pins 40, cap D, and rotor (particularly burtube 15) are keyed together, the upper section 32 of the wrench may be turned one way or the other to adjust the collet chuck of the handpiece.
Such wrench is still in widespread use, its popularity being attributable at least in part to the relatively compact design which permits it to be gripped between the fingers for one-handed wrench operation. While such wrench has significant advantages over prior constructions and is not difficult to manipulate properly, it nevertheless requires the exercise of some skill for correct wrench operation. Normally, the user grips the wrench by holding upper section 32 between the fingers of one hand and then guides shaft 31 into the head of the handpiece through the central opening in cap D. When pins 40 engage the cap's top surface, the user proceeds to turn the wrench in his fingers until the pins enter the outer openings in the cap. If, in an effort to hasten the entry of such pins into such openings, the user exerts an axial force on the wrench, the increased frictional resistance between the tips of pins 40 and the cap's top surface tends to resist sliding movement of the parts and may have a negative effect in hastening entry of the pins.
After the pins have been inserted into the outer openings of the cap, the user simply rotates the upper section 32 until the handpiece rotor assembly has been turned sufficiently to bring the apertures of the rotor into alignment with the cap openings and pins. The pins 40 may then enter the apertures 43 of the rotor and lock the rotor, cap, and lower wrench section 33 against relative rotation. Thereafter, rotation of the upper section 32 causes tightening or loosening of the chuck.
Ordinarily, the force required in such a tightening or loosening operation may be applied with little effort; however, if the chuck of the handpiece has been overtightened in a previous operation, a proportionately greater rotational force must be applied to the upper section 32 to turn the chuck and release the dental bur. Should such greater rotational force be applied to both sections 32 and 33 of the wrench, rather than just to upper section 32, the user may find that he has inadvertently unthreaded cap D from the head C of the handpiece instead of effecting a release of bur E.
Inexperienced or careless wrench operation may result in other problems as well. FIG. 4 of U.S. Pat. No. 3,325,899 shows a small but definite spacing between the underside of wrench F and the top surface of cap D. Such spacing, to a greater or lesser extend, is necessary in order to insure bottoming of pins 42 in rotor apertures 43. However, such spacing also gives rise to the possibility that should the wrench be misused, especially in a way that involves the application of excessive force in either a tightening or loosening operation, the wrench might be canted out of axial alignment with the handpiece, causing a bending of pins 40 or shaft 31 of the wrench and possible serious damage to the handpiece itself.
The possibilities of misalignment may be reduced somewhat by wrenches of later design, such as those disclosed in Lake-Jaremus U.S. Pat. No. 3,888,008 and Junkel-Kneipper U.S. Pat. No. 3,935,761, but at the expense of increased bulk and at least apparent increased complexity. The wrenches of both of these patents are directed to systems in which the wrench operates to hold the chuck (rather than the rotor) stationary while the rotor (not the chuck) is turned. While it is possible to hold and operate the wrench of U.S. Pat. No. 3,935,761 in one hand, dentists commonly regard such wrench as one requiring two-handed operation--one hand for the purpose of holding the wrench and maintaining the head of the handpiece within shroud 15, and the other for rotating wheel 27. Neither of these patents, unlike U.S. Pat. No. 3,325,899, involves a wrench which utilizes the relatively simple but highly effective concept of securing a rotor against rotation by keying it to the cap of the handpiece through the use of pins which extend through the cap and into the rotor.
Other patents concerned with handpiece wrenches and disclosing the state of the art are U.S. Pat. Nos. 3,947,966, 3,960,039, 4,015,335, 4,020,556, and 3,499,223.